Light sensitive electric discharge device and control circuits therefor



Aprifi 318, 195C E. D. M c/ 2 5134 71133 LIGHT SENSITIVE ELECTRIC DISCHARGE DEVICE AND CONTROL CIRCUITS THEREFOR Original Filed Sept. 24, 1942 2 Sheets-Sheet 2 BREAKDOWN Si MIIHO AMPS.

LIGHT SOURCE OFF 5.0

E 1110 u 0 E was PHOTO-EMISSI VE SURFACE 43 PHOTO- EMISS/VE SURFACE A GETTR SUPPORT 70 53 Inventor: Elmer D. Mc Arthur, b fiwwyfxlw w y His Attorney.

i atenteci Apr. 18,

LIGHT SENSITIVE ELECTRIC DISCHARGE DEVICE AND CONTROL CIRCUITS THERE- FOR Elmer D. McArthur, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Original application September 24, 1942, Serial No. 459,532. Divided and this application September 11, 1944, Serial No. 553,528

9 Claims. (Cl. 250-211) My invention relates to light sensitive or photoresponsive electric discharge devices and control circuits or systems therefor. This application is a division of my Patent No. 2,434,622, issued J anuary 13, 1948.

Heretofore, in photo-sensitive or photo-responsive circuits where it has been desired to effect a circuit controlling operation in response to light intensities, it has frequently been necessary to employ a conventional type photo-emissive electric discharge device with a highly sensitive voltage or current means, such as an electromagnetic relay, in order that sufficient actuating current or power is obtained tocarry out the intended circuit controlling operation. For example, where it has been desired to actuate a mechanism of appreciable size, it has frequently been necessary to employ a highly sensitive electromagnetic type relay or equivalent device, resulting in a relatively expensive system which lacked the desired degree of flexibility in control or adjustment.

It is an object of my invention to provide new and improved light sensitive control systems.

It is another object of my invention to provide new and improved photo-emissive or light sensitive electric discharge devices.

It is a further object of my invention to provide new and improved photo-emissive or light sensitive electric discharge devices of the type employing ionizable mediums.

It is a still further object of my invention to provide new and improved electric discharge devices of the photo-emissive type wherein a high degree of sensitivity is obtained by electrode construction and configuration such that the thermal effect of the thermionic cathode upon the photo-emissive control member is substantially negligible. .It is a still further object of my invention to provide new and improved light sensitive or photo-emissive electric discharge devices of the controlled type which employ an ionizable me- :dium and which arecapable of operation in a manner so that the anode current is accurately controllable or adjustable within a region not eiiecting ionization of the medium and which also permits conduction of current between an anode and a cathode by an arc discharge when either the intensity of the light impinging on the light sensitive surface attains a predetermined value, or when the control member voltage is raised with respect to the cathode.

.Briefly stated, in the illustrated embodiments of my invention I provide new and improved electric discharge devices of the light sensitive or photo-emissive type which comprise an anode, a cathode, and a photo-emissive or light sensitive electrostatic control member enclosed within an atmosphere of an ionizable medium such as an inert gas. The electrode configuration is such that the thermal effects of the thermionic cathode upon the photo-emissive control member are substantially minimized, thereby not appreciably affecting the emission characteristics of the control member. In this manner a greater sensitivity of control is obtained than that afforded by arrangements provided heretofore.

In accordance with a still further feature of my invention, I provide in light sensitive electric discharge devices of the above described type a heat shield which may be metallic and which is positioned between the thermionic cathode and the photo-sensitive control member, thereby absorbing a large percentage of the radiation and convection heat incident to the thermionic cathode and rendering the control member emission characteristics substantially independent of cathode temperature.

In accordance with a still further feature of my invention, I provide new and improved control circuits for electric discharge devices of the above-described type whereby appreciable power or current is available for the actuation of electromagnetic devices such as relays, etc. In accordance with a still further feature of my invention, I provide new and improved light sensitive electric discharge devices of the controlled type having a photo-emissive electrostatic control member the potential of which permits control or adjustment of the anode current for given values of light intensity impinging on the control member within a region of operation which does not effect appreciable ionization of the medium surrounding the electrodes. The potential of the electrostatic control member may also be controlled so that the medium does become ionized to establish an arc discharge between the anode and cathode in the event the intensity of the light impinging on the photo-emissive surface of the control member attains a predetermined value.

For a better understanding of my invention, reference may be had to the following description taken in connection with the accompanying drawings, and its scope will be pointed out in the appended claims. Figs. 1 and 2 diagrammatically illustrate one embodiment of my invention which is a light sensitive electric discharge device of the controlled type; Figs. 3 and 4 diagrammatically illustrate alternative forms or configurations of the photo-sensitive electrostatic control member; Fig. represents diagrammatically a light-sensitive system embodying one feature of my invention whereintheanode current of the discharge device may be controlled or adjusted within two separate regions, that is with in a region where ionization of the medium is obtained and a second region wherein ionizationis not produced. Fig. 6 represents; certain optlating characteristics of my invention, particularly the system shown in Fig. 5. Figs. 7-1 inclusive. diagrammatically illustrate stilljfurtherlight sensitive systems embodying features of'my invention; Fig. 11 diagrammatically represents a still further embodiment of my invention-whereinthe electrode configuration is of'diiierent formuthan that illustrated in Figs. 1 and 3, and Fig. 12'illustrates a still further modification wherein a;

mesh type anode and a heat shield are provided.

Beferringnowto Figs. 1 and 2 of the drawings, my: invention isthere .illustratedas comprising a light sensitive or, photo-sensitive electric discharge device ofthecontrolled type including a plurality of electrodes mounted within a glass receptacle or bulb l mountedion a base member, 2.;andwhichacts as a support for the discharge device as awhole. The bulb l may containan ionizable. medium, such as .an inert gasatthedesired low pressure, and this operation may.v be effected by filling. the exhausted bulb with a gas. through a suitable glass tubulation (not shown) which is sealedoi'f and which may lie Within the base 2. Forexample, I may empl'oyl an atmosphere of argon at a pressure of substantially 1.00 to 150 microns. It will be ap-- preci-ated. that Imay employiother inert gases spchas neon and xenon, etc. if desired, I-provi'de within the enclosure of the bulb l a thermi onic. cathode 3-of elongated form andwhich may be constructed of an electron emissive materials ich as tungsten, thoriated tungsten, or the like. Alternatively, the. cathode 3- inay; be v constructed, of a metal, such as nickel, which is coated with an alkaline earth metal t or an oxidethereof to increase the electron emissivity. A cathode heatingeelement, (not. shown) extends-into the intenor. of the cylindrical-. cathode 3,. and cathode heatingeelementterminals 5. and 6, alsoshown in Fig. 2, are provided. Aglass pressilsupports and seals. conductors 8 and?! which are connected to terminals 5;-and, 6. A cathode conductor to ,is also supported ,by-and sealed, in the; gilassj'press l and. is terminated-:inthe metal1ic;

prong or conductor Hwhich-is also attached to.

the. baseZ.

I..provide an .anode l2 which may be of elongatedconfiguration, such as a metalliccylinder or frod having-its principal dimension or-lengthsubstantially parallel with that of the thermionic cathode 3 and which is ofrelatively small area. A conductor and supporting member I 3 for the anode ,IZ is supported by and sealed inthe glasspressIand isterininated in an externally accessibleconductor, l3 attached to the base 2-. Of, course, it is to be appreciated-thatother forms; of anode structure may be employed.

Inorder to control. the anode current and to provide means which is light-sensitive, I'provide a. photo emissiveelectrostatic. controLmember ,I 4z-. which is I connected in a spaced .relationbetween thecathode 3 and theanode-IZ and may be sup.- ported by a, stem orcond-uctorl5-which is. in turn supported by-and sealed in theglass'press; 1 and made externally; accessible. by means. of v a.

conductor !6 which is supported by the base 2. A rigid construction of the control member I4 and the supporting means therefor may comprise supporting members ll and I8 which may be welded or soldered to the control member. l4 and one of which, such as supporting member 1-8, may be attached or welded to conductor l5 and the other of which may be supported by the member l9 which is embedded or anchored in theglass press};

The photo-emissive electrostatic control member l4 iscoated with av material to provide a high degree of photo-emissivity For example, the electrostatic control member [4 may comprise an oxidized silver base over which a layer of caesium is applied. Of course, other photo emissivemetals or alloys may be employed. The control member M in one modification of my invention is preferably of semi-cylindrical configuration being concave to the anode l2 and convex toward the cathode 3. Furthermore, the area of: the control member, I4 is great as corn-- pared with that of the cathode 3. so-that the; thermal effects of the thermionic cathode. Jupcn; the emission characteristics and control charac teristicsof the control member l4 are-.substan tially negligible. Stated in other words, the PTO-1* jected area of ;the cathode -3 upon the :SlIIfaCBJ-Ot' the control member is very small.

The, light sensitive surface of; control member Hi. may.v be placed upon the concave portioniofi control member l4 facingthe anode l2 and may. be'depcsited thereon in the-usual or conven.-- tional manner. Onedetailvl'ewof the'formrof the. control member I4\is= illustrated in:Fig.-.3.; where the portion of 1 the control member nearest the cathode 3 is provided'witl' v an elongated aperej ture 26 which issubstantially CO-BXtBl'lSiVBWith; theprincipal dimension of the cathode 3.v When StlChz aconfiguration of;v the control member. is; employed, it-willbe appreciated-that some means; is required to support the two sections of;the control. member and to accomplish-thisresult I may employsemi-circular conductors 2! and '22;- which are welded or solderedto the control memberm.

An alternative form of-thephoto-emissive electrostatic control.- member; is. shown in Fig.1 4 wherein a semi-cylindrical control member 23;,-is; provided with a series :of vertically-spaced apere. tures 24 in a line which issubsta-ntially: co:exten-- sive,withthe principal dimension or: heightzof the thermionic cathode 3;

It! will be appreciated that in :both :0f the :ar rangementsillustrated: in Figs; 3 andr t; the.pro:+.- jected areaof they-thermionic cathode-onthe: photo-sensitive control} member" is: relatively small,- approachingrzerozas arlimit :inthe 'arrange merit shown in Fig. 3. In this manner, thetheremal eflectof; :thecathodeeonthe 'control member are minimizedgand at the-:same time: thereisprovidedv an appreciable path; for the-arc-di's charge which is:- established;' under certain con-- ditions; of-v operationabctweerrathe. anode" l2 and;

the, cathode. The; operation oftheemb'odi-zment of=my inventionillustrated'in'Figs. land- 2 tural features described above with. particular reference to Figs.--.1'and"2.-' In the arrangement: of Fig.- 5; the-electric dischargedeviceZS is illustratedqas comprising an; anode 26; a thermionic i cathode 21 having a heating element 28, and a photo-emissive electrostatic control member 29 which may be subjected to light of different intensities derived from a source which is diagrammatically illustrated as comprising a lamp 30.

The cathode heating element 28 may be energized from any suitable source of current such as a transformer 3i energized from an alternating current supply circuit 32. An adjustable biasing potential, such as a negative unidirectional biasing potential, ma be impressed upon the control member 29 by means of a battery 33. The output circuit of the discharge device 25 which comprises the anode 26 may be energized from a suitable source of current, such as a battery 34, to supply various amounts of unidirectional current to a load device 35 in accordance with the conditions of operation controlled by the discharge device 25.

Although the output circuit of the discharge device 25 may be connected in the manner illustrated in Fig. 5, that is connected across the anode 26 and the control member 29, it will be appreciated in view of the various modifications of my invention explained hereinafter that the output circuit may be connected across the anode and cathode.

Apparatus embodying the embodiments of my invention may be operated in several ways. One way in which electric discharge devices of the type described above may be employed is by the adjustment of the negative unidirectional biasing potential to effect current flow between the anode 25 and the control member 29 of adjustable or controllable value in response to the intensity of the light impinging upon the photo-emissive surface of control member 29. That is, by the adjustment of the magnitude of the biasing potential the anode current may be established to have different values, depending upon the mag-- nitude of the biasing potential. Within this region of operation, the medium is ionized but not suiliciently to produce an arc discharge. Furthermore, the magnitude of the biasing potential may be adjusted so that the current within the discharge device varies as a function of the intensity of the light impinging on the electrode 29.

'A still further way in which the electric discharge device may be operated is by establishing the magnitude of the unidirectional biasing potential to a value which, acting in conjunction with a given intensity of impinging light, effects ionization of the medium and establishes an arc discharge within the device.

The various operating characteristics of electric discharge devices constructed in accordance with the principles of my invention may be more fully appreciated by referring to the character-. istics illustrated in Fig. 6 wherein the solid portion of curve A represents the variation of the anode current 6 in of the discharge device 25 for a given intensity of light applied to the light sensitive surface of control member 29. The dotted curve B represents the amount of current which would flow between the control member 29 and anode 2e with no light present. It is emphasized that upon application of light to the control member 29, the magnitude of the current which flows is substantially increased but may be adjusted throughout an appreciable region within which the electron flow is not suflicient to efi'ect ionization of the medium by collision with the ions of the medium. The dotted extension A of curve A represents that region of operation operation of relay 42.

within which the medium becomes ionized to effect the establishment of an arc discharge. This type of operation may be effected by a relative decrease in the magnitude of the unidirectional biasing potential, or by an increase in the intensity of light which impinges on control member 29, or may be obtained by the joint action of two such controlling influences. For example, referring to the characteristics shown in Fig. 6, ionization of the medium occurs when the bias potential impressed 0n grid 29 is reduced to a value somewhat less than 1 volt for a given intensity of light impinging upon the control member. It is apparent that if the bias potential is maintained constant and the intensity of the light increased, the breakdown or the ionization. of the medium would occur at a point determined by the joint action of these two quantities.

With a plate voltage of substantially twenty volts and a negative biasing potential of three volts for the characteristics illustrated in Fig. 6, I have also found that the medium becomes ionized to efiect an arc discharge for a particular intensity of light employed.

Fig. 7 illustrates a still further embodiment which may employ electric discharge devices of the type described above, and is diagrammatically illustrated as including an electric discharge device 36 comprising a thermionic cathode 3?, an anode 38, and a photo-emissive electrostatic control member 39. These latter mentioned elements may have configurations similar to those shown in Figs. 1 and 2 and may also take the form of the elements shown in Figs. 11 and 12 to be described presently. I provide means for impressing across the anode 38 and cathode 3? of the discharge device 36 an alternating potential which may be furnished by means of a transformer 4!! energized from an alternating current circuit 4|. A load device, such as a relay 42, may be connected to be energized in accordance with the anode-cathode current and may comprise an. actuating coil 43 and contacts 44 which are connected to control a circuit 45. In order to maintain the current through the coil 43 at a substantially constant value so long as the operation of the discharge device 36 dictates energization thereof, I may connect a capacitance 46 across the actuating coil 43.

A suitable biasing potential may be impressed on control member 39 and may be of the selfrectifying type comprising a parallel connected capacitance 4i and a resistance 48. These latter mentioned elements serve to produce a negative unidirectional biasing potential during each inverse half cycle of the anode-cathode voltage due to the control member rectification charac-- teristic so that at the beginning of each positive: half cycle the discharge device 3c is in a condition to discriminate as to the intensity of the: light which impinges upon its photo-emissive surface.

One way in which the arrangement of my invention shown in Fig. '7 may be employed is as a means for detecting various intensities of light and for operating the relay 42 when the light intensity attains a predetermined value. So long as the light intensity remains below a predetermined value, the negative biasing potential established by capacitance 4i and resistance 48 is suihcient to maintain the electron flow from the control member 39 to sufficiently below that which effects ionization of the medium and below that which effects the anode at a value However, if the photo v hc actuatins-cml 13 each positive; half ,1

cycle of; voltage .appl ti to ancde...3E--, the. capacitance 416-; serves to maintain themurrent through-the. .actuatingmoil substantially constant eyen du-ring negative. half .rcycles. However, if

l A i r v \vl chQeff-ectsst? iatin..01" discharges durin each p sit /e.,;ha1f zcycle, .the..:ano.de.- cathode currentrflcwn llanal;:continueanoithatcharge oi capacitfihfie ASL-Will ibesdi'ssipatedzcausing. coil Ali to be deenergizedz-or sini -iciently reduced in I energiza tion, thereby opening contacts .44;

A -i fi l l f hmlr odificationoii in." invention ss li strate zin respects to that Fig. 7 and rcorresponding elements have. been assigned. like reference numerals, 1112 116 modification ,0f ;.6-ig. 81a separatesource -oi-:.biasin pctentiah such. as. an alter current; source. :59; isemployed to .im-

Di S 5 asin :pQtentiaLacros control member.

3.918 1 9 Cathode 31,: through a transformer. 5% The,- alternatin bias-iyolta'ge ispreierably. ad+

justed to belBO electricalrdeercesout of phase.

with theganode cathodevoltage, and the. secondary winding; of transformer. this is connected i in series, relation with a capacitance Durinaeacinverse-half cycle of anode-cathode-Voltage; the-voltage produced by transformer..- 56 -establishes; :a; hia ug. potential on control.

member 39 ;by-,char the capacitance. 5i the current fOIgDlfOQit y Vi tue ;0.ithe;elcctron emission-current which flows :between control rus ation :and; cathode. increases 931m". 'ssiye. current;

3]. e .If the intensitycoi t" ficiently to increase the; .phptm to. a val ue which efifii ts ion-ization of .the. medium m ss ablish s 3g} ,bec0mes p ing energization of the load de iice.

actuating 0011. 52. In this manneigitpwill beappreciated that thenegative biasing poten ti al established during. each negative hali cycle large ofcapacitance 5!, facilitates establishmentor the. arc discharge withindeviceiistl. Thisttype of operation willlbeefiected when the increase of;

of; anode-cathode voltage. The

light intensity occurs during a positive halilcycle or during a negative. half cycle or applied anodecathode voltaga. When the initiationof the increased light intensity occurs duringa negative.- half cycle, the ionization effect issuflicient to.

carry over into the succeeding positive half cycle so that current flow is produced in the anodecathode circuit.

A further embodiment of my invention is. illustrated-in Fig. 9 whichis similar in many respects to Fig. '7, and-corresponding elements have been assigned like referencenumerals. In-

stead of employing an alternating current cir-.

cult for establishingthe voltage difference, be-

tween thecathode 3i and the control member 39,I may employ a unidirectional source such as a battery 52. In this arrangement, when the light-intensity-attains a predetermined yalue, an

arc discharge will be initiated and current flow will be established in the anode-cathode'circuit;

a es to. ivalueehelow that.

. hvwhichs is ssirniiar in many.

inggthis chargeibei-ng obtained areeqwhcu theansid t v I} t v ltS; CyG1iC-} variationin; p otenti al,-, anode-cathode current avill flow w .i l'imcy One of theprincipalfeatures 'ot'li'ght sensitive electric discharge devices built in accordance with m y-invention is that the controlcircuit which establishes thesensitivity Of"the:ini-tia$10n of operation maytbesa lowimpedance circuit and notionswhich functions principally iinwresponse to-potential; By "the -..provision .oi- .suclr 1a..- devicev and rsystem it possible-to obtain greater -*'de'- grees-zof, rsensitivityrthan thatiafiorded lbyvmauy otttheiprior --art:arrangements particularly whcre it =.is :desired tOfBmp1Qyith8.;de!/i(3e and- 13118 system in aehmateawhichsis asubjccted to: extreme-variations in. atmospheric conditions such". as mariations inirelativechumidity and temperature which would tend-ate produce condensation of :moistureort high: voltageelements comprising the sensiti've part of. thew-circuit; thereby creducing its sensitivity;

In Fig. 10 I have diagrammatically iillustrateda stillx further embodiment c. of imy i. invention wherein la.v low; impedance control I circuit .53 'is connected :across. .the control member and: cathode 3:1; or electric discharge. device-- 36; the initiation .oi. 'the arc;.;discharge or operation 'ot the device 3 being effected-shy thexcurrent: which flows between control member .-39"and cathode: 3?. That is, the circuit is primarily a current. responsive c'rcuit1.and wdoes lnot zdepend appreciably upon large voltage. variations, thereby .dispensing ui-itlrthe. employment :otzimpedance elemerits of relatiytely high-value ..which:.in .turn effect .or. .acause -the presenoe ;.of z-relatively highvoltages.-

Fig. 11 diagrammatically illustrates a :still fur--- then modification-bf. my invention, particularly a new and.qimproved-glight :sensitiue electricidiset ha edeviceg hichiemploys'anionizablemedium; s c as anlziner as; a d a plurality of electrodes including ;an;electrosta:tic rphotoeemissive control; mb eicrring .particularlyto the electric discharge device show-1min; Fig; 11,, it comprises an.-;enc1o s -ins :ifeceptacle such, as. a glass bulb. r54. ifih may dnc u ea i drical y lassmess s unner d bran 1 2se-.;56--1. I provide a. phot emissive electrostatic control member 58 which? r undsean appreciab e; mc siomof za r-w re anode .51. ,and,;which;:may -,be; ran. inverted trunnca jc onm ar nse -.-phc m ssive surface; on:

th conc e po tion:t e eorz acins he-anode.-

The control member 58;;is constructed to haven; w p n a5$ throu h chrthe ancde'51 ex Anode 51and cqntrol'm mh r "58 maybe.

tends. sunnq t di ov he.., -ass-;nre.ss z y; p ieht Mi -6,11,, and; 62; respectively. I viinorle..51, and l 419-, ri ht i may besupn rt d- -bya fv tre usi r glass s1eeve.or.pencil 60, in which these me-mbers and: conductors (Share ,embeddedorsealed. The .up rights. ,6 i, and ,6 L areconnectedto prongs 63 and a 64 &through, conductors 5. and ,66 respectively.:

which may be scaledin thei glassvpresssgiik.

I provide a thermionic .cathodelpreferably; conueted. oi a metallic..ribbonfii. having them:

ionic emissiyecharacteristics and which may 1- have, a substantially circular confi uration. lying:

n .pla e..su t ntiallr perpend ular to the p ic angifl'l which are supportedby base. 56..., It iwill l tfidli a ithc rojected areaof .cathode..r b-..- n '6 P9nhe. facc. ofihecon member,

58 is inappreciable so that the thermal eflects of the cathode do not materially affect the emission characteristics of the control member 58.

In operation, the embodiment of my invention shown in Fig. 11 functions substantially the same as that explained above in connection with Figs. 1, 2, and 5.

In the modification of my invention shown in Fig. 12, which is similar in many respects to that shown in Fig. 11 and corresponding elements have been assigned like reference numerals, I provide a heat shield in spaced relation between the thermionic ribbon cathode 61 and the photo-emissive electrostatic control member 58. This heat shield may be constructed of metal such as nickel or copper having an appreciable thermal capacity to absorb heat which is radiated from the cathode 61. The heat shield may comprise a metallic cylinder 12 supported upon a pencil or glass stem 13 and may be sealed thereto by employing an alloy of iron-nickel-cobalt. The portion of the shield which extends toward the control member 58 may comprise a flared or flanged section including a metallic annular member 14 which lies in a plane substantially perpendicular to the direction of heat flow or heat radiation between members 6'! and 58. Conductors Ti and 18, which are connected to anode I and control member 58, may extend through and be sealed in the glass stem 13, and may be made externally accessible by means of a base (not shown).

In the modification of my invention shown in Fig. 12, I provide an anode which may take the form of an inverted cone l5 and may be constructed of a wire mesh or grid and which is supported at the vertex thereof by means of an ."s'

upright 16 which extends through the lower opening 59 in the control member 58. The concave portion or surface of the control member 58 is also coated with a photo-emissive material and the initiation of current between the anode and cathode may be accomplished in the same manner, as explained above, in connection with the devices shown in Figs. 1, 2, 5, and 11.

While I have shown and described my invention as applied to particular systems employing various devices diagrammatically shown, it will be obvious to those skilled in the art that changes and modifications may be made without departing from my invention and I therefore aim in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States, is:

1. A circuit comprising a light sensitive electric discharge device including a plurality of electrodes enclosed within an ionizable gaseous medium, said electrodes including an anode, a cathode and a photo-emissive electrostatic control member for controlling the flow of current between said anode and cathode, a source of alternating anode voltage connected between said anode and cathode for causin an anode current to flow in said device, means connected to said control member for establishing and applying 6 tioned at values insufficient to establish an ionized gaseous discharge in said discharge device, and photo-emissive means affixed to said control whereby the potential of said control member may be raised responsive to the impingement of light thereon to a value sufiicient to establish an ionized gaseous discharge in said discharge device.

2. A circuit as in claim 1 in which said first mentioned means comprises a capacitor connected to said control member.

3. A circuit as in claim 1 in which said first mentioned means comprises a resistor and a capacitor in a parallel circuit connected in series with said control member.

4:. A circuit as in claim 1 including relay means selectively responsive to an ionized gaseous discharge in said device.

5. A circuit comprising a light sensitive electric discharge device including a plurality of electrodes enclosed within an ionizable medium, said electrodes including an anode, a cathode and a photo-emissive electrostatic control member, a source of alternating anode voltage connected between said anode and cathode for causing an anode current to flow in said device, a source of alternating control voltage and means responsive thereto for establishing and applying to said control member a self-biasing voltage connected to said control member for controlling the flow of said anode current, said self-biasing voltage and said anode voltage being mutually proportioned at values insufiicient to establish an ionized discharge in said discharge device, and photo-emissive means aflixed to said control member whereby the potential of said control member may be raised responsive to the impingement of light thereon to a value sufiicient to establish an ionized discharge in said discharge device.

6. A circuit as in claim 5 in which said first mentioned means comprises a capacitor in series with said source of alternating control voltage and said control member.

7. A circuit as in claim 5 in which said first mentioned means comprises a resistor and a capacitor in a parallel circuit connected in series with said control member.

8. A circuit as in claim 5 in which said alter- 3 established by flow of current between said cathode and control member during inverse half cycles of said anode voltage.

9. A circuit as in claim 5 including relay means selectively responsive to an ionized gaseous discharge in said device.

ELMER D. McARTHUR.

REFERENCES CITED The folloving references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,522,070 Nakken Jan. 6, 1925 1,755,272 Ruben Apr. 22, 1930 1,880,092 Hull Sept. 27, 1932 2,145,180 Friederich Jan. 24, 1939 

